The invention relates generally to insulated piping and, in particular, to a cryogenic seal for a vacuum-insulated pipe expansion bellows.
Thermally insulated pipes have a wide variety of industrial applications. For example, insulated piping is used to transport cryogenic liquids, that is, liquids having a boiling point at a temperature below −150° F. at atmospheric pressure, between storage tanks or between a storage tank and a use device. Other examples include utilization in chemical plants or petroleum refining.
A major source of thermal leakage in insulated pipe systems is at the junction between two pipe sections. It is therefore preferable to eliminate such pipe joints. This would require, however, that the insulating piping be custom-manufactured to fulfill the length requirements of specific applications. Such an arrangement would be prohibitively expensive. As a result, efforts have been directed towards developing prefabricated insulated pipe sections that may be connected with joints that suffer from minimal thermal leakage. An appropriate number of such sections may be delivered and joined in the field to create efficient piping systems of desired lengths.
One type of prefabricated pipe features vacuum-insulated sections. This system is offered by Chart Industries, Inc. of Garfield Heights, Ohio under the name VIP. A VIP pipe section is constructed of an inner pipe surrounded by an outer pipe. The inner pipe and outer pipe are concentrically positioned so that an annular insulation space is formed therebetween. The ends of the outer pipe are welded to the inner pipe so that the annular space is sealed. Either the inner pipe or the outer pipe is provided with a bellows so that the integrity of the welds is preserved when the pipes expand or contract by differing amounts due to temperature changes.
The insulation space of the VIP pipe is evacuated and filled with a multiplicity of layers of thin fabric formed of fine fibers of glass, cellulose or other fibrous material characterized by low heat conductivity. Positioned between the thin fabric layers are reflective barrier sheets formed of highly reflective material, such as an aluminum foil. When utilized in cryogenic liquid applications, the insulation arrangement of VIP pipe minimizes conduction and radiation heat gain to the cryogen within the inner pipes.
VIP pipe sections may be connected by brazing or butt-welding with the joints covered by insulated clam shells. VIP pipe sections, however, are optimally joined by a bayonet connector arrangement such as is disclosed in U.S. Pat. No. 4,491,347 to Gustafson. The '347 patent discloses pipe sections that have interfitting metal male and female end portions. The material of the female portion has a coefficient of expansion that is greater than that of the male portion. As a result, when cryogenic liquid flows through the pipe sections, the female portion contracts into sealing relationship with the male portion.
Vacuum-insulated pipe such as the VIP offer high performance with an overall heat loss of only 160 BTU/hr for 100 feet of 1 inch pipe when liquid nitrogen is carried. In addition, vacuum-insulated pipes such as the VIP typically retain their original insulating capabilities for fifteen to twenty years.
A disadvantage associated with existing vacuum-insulated piping is that failures of the bellows of the piping can cause cryogenic material to leak from within the inside pipe into the vacuum area between the inner and outer pipes. Vaporization of the leaked liquid cryogenic material and the cooling effects of the cryogenic material on the outer pipe can create a situation where other parts of the pipe spool system may fail catastrophically. Additionally, leakage of cryogenic material from the inside pipe results in safety concerns and the loss of saleable product. It is therefore desirable to provide a cryogenic seal for vacuum-insulated pipes that eliminates or minimizes such leakage.